Organic electroluminescent devices are attracting attention as candidates for flat displays of exceptionally high luminance and researches and development works directed to these devices are getting active. An organic electroluminescent device has a structure consisting of a light emitting layer put between two electrodes and a hole injected from the anode and an electron injected from the cathode recombine in the light emitting layer thereby emitting light. Organic materials useful for the devices include low-molecular-weight and high-molecular-weight materials and both materials have proven to yield organic electroluminescent devices of high luminance.
There are two types for such organic electroluminescent devices. One type uses a charge transfer material to which a fluorescent pigment is added as a light emitting layer (C. W. Tang et al., J. Appl. Phys., 65, 3610 (1989)) and the other type uses a fluorescent pigment itself as a light emitting layer (Jpn. J. Appl. Phys., 27, L269 (1988)).
Those organic electroluminescent devices which use a fluorescent pigment itself as a light emitting layer are further divided roughly into three types. The first type has a three-layer structure in which a light emitting layer is put between a hole transport layer and an electron transport layer, the second type has a two-layer structure in which one layer is a laminate of a hole transport layer and a light emitting layer and the third type likewise has a two-layer structure in which one layer is a laminate of an electron transport layer and a light emitting layer. Constructing organic electroluminescent devices in two or three layers has been known to improve the light emitting efficiency.
The electron transport layer in any of the aforementioned organic electroluminescent devices of various structures contains an electron-transmitting compound and performs a function of transmitting electrons injected from the cathode to the light emitting layer. The hole injection layer and the hole transport layer contain hole-transmitting compounds and perform a function of transmitting holes injected from the anode to the light emitting layer. Putting the hole injection layer between the anode and the light emitting layer makes it possible to inject a larger number of holes to the light emitting layer in a lower electric field and, in addition, to confine the electrons injected from the cathode or electron injection layer in the light emitting layer thereby improving the light emitting efficiency and yielding organic electroluminescent devices of excellent light emitting performance.
The performance of these organic luminescent devices, however, has not been satisfactory to warrant practical use. A lack of durability of the materials used for the devices, in particular, poor durability of the hole transport materials, can be cited as the largest obstacle to practical use. The existence of a nonuniform area such as grain boundary in the organic layers of an organic electroluminescent device is likely to cause the electric field to concentrate in this particular area thereby leading to deterioration and destruction of the device. For this reason, the organic layers are frequently used in the amorphous condition. Moreover, an organic electroluminescent device is a device of current injection type and any material to be used for the device is required to have a high glass transition temperature (Tg) because the device in drive deteriorates by the Joule heat if the Tg of the material is low. Another obstacle to practical use has been an unsatisfactory hole transporting performance of the hole transport materials and an impractical light emitting efficiency of the devices.
A large variety of materials, predominantly triphenylamine derivatives, have been known as hole transport materials for the aforementioned organic electroluminescent devices, but few of them are suited for practical use. For example, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-4,4′-diaminobiphenyl (TPD) reported in Appl. Phys. Let., 57, No. 6, p. 531 (1990) was poor in thermal stability and presented a problem regarding the life of devices and the like. Many triphenylamine derivatives are described in U.S. Pat. No. 5,047,687, 4,047,948, 4,536,457, JP 632307(A1), JP 5-234681(A1), JP 5-239455(A1) and JP 8-87122(A1), but none of them has satisfactory properties for practical use.
The starburst amine derivatives described in JP 4-308688(A1), JP 6-1972(A1) and Adv. Material, 6, p. 677 (1994) and the compounds described in JP 7-126226(A1), JP 8-48656(A1) and Journal of the Chem. Communication, p. 2175 (1966) do not possess the properties essential for practical use, that is, high light emitting efficiency and long life. Moreover, the use of N,N′-dinaphthyl-N,N′-diphenyl-4,4′diaminobiphenyl (NPD) is reported in JP 2851185 while the use of derivatives of NPD is reported in JP 9-194441(A1); an improvement over TPD is described in either case, but even their use did not yield satisfactory hole transport performance and heat resistance.
As noted above, the hole transport materials used in the conventional organic electroluminescent devices do not perform satisfactorily to warrant practical use and there is a demand for enhanced efficiency and longer life for organic electroluminescent devices by the use of improved materials. Furthermore, in the majority of organic electroluminescent devices, light is emitted mostly from the light emitting layer or electron transport layer provided separately from the charge transport layer and rarely from the hole transport layer. One reason for this is the problem of compatibility of the hole transport layer with the simultaneously used electron transport layer, but the factors just as important are the color and intensity of the light emitted by the hole transport material itself. In spite of the expectation that emission of light from the hole transport layer, if possible, would increase the practical value, there are available few such materials. Some of the materials of this kind, represented by the 9-anthryl derivatives described in JP 9-157643(A1), have problems because they emit light of long wavelength and cannot emit light of short wavelength in most cases.
This invention has been made in consideration of the problems associated with the aforementioned conventional technologies and has an object of providing organic electroluminescent devices of high light emitting efficiency and long life and also providing novel compounds, hole transport materials and organic electroluminescent materials useful for said devices.